Carbon-based materials, although they are fragile and have poor oxidation resistance, have some outstanding characteristics. For example they hardly get wet with molten metals and, by virtue of such characteristics, they are used as parts of metal melting equipment or as members of welding equipment.
Generally, these parts or members are manufactured by cutting and shaping a large block or by integrating carbon-made parts with shaped metallic bodies by mechanical jointing means, such as screwing or caulking.
However, when wholly made of carbon, they become expensive and, in addition, are fragile and break easily in many instances during use, whereas those made by mechanical integration with metallic bodies are disadvantageous in that screws, caulkings or the like may readily become loosened and allow parts to fall off.
In surmounting these drawbacks, it is effective to restrict the use of carbon to those portions in which its use is indispensable or, in other words, make main bodies of metallic materials, followed by metallurgical bonding of both. For this, however, it is necessary to solve the following problems.
(1) Problem of thermal stress on joints
Essentially, carbon and metals are extremely different in coefficient of expansion. Therefore, great thermal stress may readily be exerted on joints, causing destruction thereof.
(2) Problem of thermal fatigue
Carbon-metal composites are generally used maximally at an ambient temperature of 450.degree. C. Thus they are subjected to repeated heating from ordinary temperature to 450.degree. C. and, because of the resulting fatigue, they cannot be used for a long period of time.
Any composite made of a carbon-based material and a metallic material with which such problems can be solved has not been available as yet. Improvements in heat resistance and thermal fatigue resistance are earnestly desired.
Meanwhile, the welding nozzles so far used in arc welding, such as carbon dioxide gas welding, TIG welding or MIG welding, are, for example, torch nozzles made from copper, copper alloys, aluminum, aluminum alloys or the like, or modifications thereof plated with chromium, chromium alloys or the like, or surface-treated (cf. e.g. Japanese Patent Publication No. 56-10982 and Japanese Unexamined (Kokai) Patent Publication No. 55-10350). However, these, as such, cannot prevent spatter adhesion, hence they are generally used with an antispatter coating applied to their surface. However, such means disadvantageously involve complicated operations for spatter removal or antispatter agent application, for instance.
In view of such problems, attempts have so far been made to use, as welding nozzle materials, carbon-based materials that are excellent materials from the viewpoint of preventing spatter adhesion during welding. Thus the known methods include, among others, the one comprising forming welding nozzles wholly from a carbon-based materials, the one comprising providing rings made of a carbon-based material with a screw thread and joining the same to metallic nozzle bodies, and the one comprising mechanically caulking parts made of a carbon-based material with metallic nozzle bodies. However, each method has its drawbacks; for instance, because of the fragility of carbon-based materials, damage may readily occur during use, or screwed or caulked portions may readily get loose during use, resulting in the portions falling off.